The present invention relates to a corona discharge device which applies static charges uniformly to the surface of a sensitive (photosensitive) material and, in particular, to a corona discharge device having a control electrode.
In recent years, an organic photosensitive material has been developed remarkably and has been used in many electronic photographic apparatuses. Most of the organic photosensitive material are used in a negative charged manner. As a corona discharge device which applies charges to the surface of a photosensitive material, in general, there are used a corotron and a scorotron having a control electrode. It is known that, when the corotron and scorotron are used in a negative polarity, a large amount of NOx (nitrogen oxides), ozone and the like are produced. Such NOx, ozone and the like facilitate the oxidation of the surface of the photosensitive material, the reduction of the electric resistance thereof, and the leakage of the charges on the surface thereof and, therefore, in images formed by use of the corotron and scorotron, there are found defects such as unsharp images, running images, white removed images and the like. To avoid this, there is taken a measure to attract the NOx, ozone and the like, which are produced from the corona discharge device when it performs corona discharge, out of the corona discharge device or to ventilate the corona discharge device. In parallel with the measure, there is proposed in Japanese Patent Unexamined Publication No. Hei 2-281274 a method of applying a substance into a case of a corona discharge device which can absorb or resolve the NOx, ozone and the like.
According to the method disclosed in the above Japanese Patent Publication, a thin conductive dry film (Electrodug (a trade mark) manufactured by Achison Corroid Co., U.S.A.), which is substantially continuous aluminum hydroxide film containing graphite particles and powder nickel, is coated to a shield, and the aluminum hydroxide of the coated film combines with nitrogen oxide or the like to form aluminum nitrate, thereby neutralizing the NOx.
However, the method or technology disclosed in the above Japanese Patent Publication has the following problems.
A control electrode can be produced, for example, by arranging grids each of 0.1 mm on a stainless steel frame having a thickness of 0.1 mm at an angle of 45.degree. and at intervals of 1 mm by etching, while the control electrode has an opening ratio of about 90%. In a scorotron with such control electrode mounted thereto, a direct current (DC) voltage of -5500V is applied to the discharge electrode thereof, while a DC voltage of -580V is applied to the control electrode. If this scorotron is mounted to an electronic copying machine, then the dark potential (V-DDP) of a sensitive material becomes -600V. The BKG potential (V-BKG) of the sensitive material is set as -150V by controlling the quantity of light used to radiate a manuscript. In our test, this electronic copying machine was installed under the environment with the temperature of 10.degree. C. and humidity of 30% and then a copying operation was carried out. After copying about 15,000 sheets and elaspe of one night, in a first copy, there occurred a white removal phenomenon which seemed to be caused by the discharge products of corotron. This phenomenon is caused in the following manner: That is, when a corona discharge device which has stopped discharge is laid in a rest condition for a long time, the NOx produced by corotron discharge and absorbed in the corotron is released to denature the surface of the photosensitive material being stopped opposed to the corona discharge device, and the denatured sensitive material surface portion causes the white removal phenomenon.
In view of this, when a coat (50 .mu.m) of Electrodug (JD-29080, trade mark) manufactured by Achison Corroid Co., U.S.A. was applied to the front and back surfaces of the above-mentioned control electrode having the opening ratio of 90%, the opening ratio was changed to about 87%. When a DC voltage of -593V was applied to the control electrode, then the dark potential (V-DDP) of the sensitive material was changed to -600V. By use of this discharge electrode, a copying test was carried out under the same conditions. In this copying test, even after copying 20,000 sheets, no white removal occurred. However, after copying about 8,000 sheets, the densities of the two end portions of an image began to be deepen (this phenomenon is called a fog) and, after 13,000 copies, the densities of the two end portions reached a problem level.
When checking the variations of the BKG potential (see FIG. 10) and the variations of the sensitive material dark potential (see FIG. 11) in the axial direction of the photosensitive material in this case, the BKG potential is increased in the two end portions of the photosensitive material in the axial direction thereof when a trouble occurs, when the control electrode is replaced, and when the sensitive material is replaced, while the dark potential is increased in the two end portions of the photosensitive material in the axial direction thereof when a trouble occurs, and when the control electrode is replaced. Also, referring to the control electrode corresponding to the photosensitive material, similarly, the potentials of the two end portions of the control electrode in the longitudinal direction thereof are increased and the coated films of the control electrode corresponding to the potential increased portions were found insulated. In other words, if the control electrode is insulated, then an amount of corona ions flowing into the control electrode is reduced and an amount of corona ions flowing into the sensitive material is increased, with the result that the charged potential is increased, which is the fog phenomenon. Also, as the charged potential is increased, the density of the image is deepened, which is also the fog phenomenon. In this test, the dark potential (V-DDP) of the sensitive material was increased by 120V up to -720V.
In view of the above, the inventors tried to solve the fog phenomenon of the image by increasing the thickness of the coated film of the control electrode.
Three coats (90 .mu.m) of Electrodug (JD-29080, trade mark) manufactured by Achison Corroid Co., U.S.A., were respectively coated onto the front and back surfaces of the control electrode having an opening ratio of 90%. The reason why the number of coatings is increased from 1 to 3 is that three coats are necessary to obtain a coat thickness of 90 .mu.m because the film thickness varies greatly by thick coating. After the two films each of 90 .mu.m were coated, the opening ratio of the control electrode was changed to about 85%. When a DC voltage of -610V was applied to the control electrode, then the dark potential (V-DDP) of the sensitive material was changed to -600V. Then, the discharge electrode was tested on the same conditions as above in copying. In this test, even after copying 50,000 sheets, no white removal phenomenon occurred. However, after about 25,000 copies, the two end portions of the formed image began to fog (that is, the densities of the two end portions thereof began to deepen) and, from 45,000 copies, the densities of the two end portions reached a problem level. In this case, the dark potential (V-DDP) of the sensitive material was -730V, which means that the dark potential was raised by 130V.
As described above, if the thickness of a coated film of the control electrode is increased, then the fogging of the two end portions of the image can be improved, but not yet perfect. Also, it is difficult to achieve the increase of the thickness of a coated film of the control electrode at a time and, in fact, there are necessary a plurality of coatings for this purpose, which incurs a problem of an increased cost.
As described above, in the conventional technology in which a film of a substance to absorb or resolve the NOx, ozone and the like is coated to the interior of a case of a corona discharge device, when the coated film of the control electrode is thin, then occurrence of a white removal phenomenon can be truly restricted but a greater secondary problem arises, that is, the two end portions of the image are fogged. To prevent the fogging phenomenon, it is necessary to increase the thickness of the coated film of the control electrode. However, our test shows that, even if the thickness of the coated film is increased, the fogging of the two end portions of the image occurs when the number of copies increases. Also, there arises another problem that the cost of the device is increased because it is necessary to increase the thickness of the coated material.